This invention relates to a display terminal for a digital data processing system, and more particularly to increased resolution of dot-matrix display of alphanumeric and other special characters on a cathode ray tube (CRT) operated in a raster scan mode, as disclosed in U.S. Pat. No. 3,345,458.
In the raster scan mode, the electron beam is swept across the screen in parallel lines until the entire surface (frame) of the screen has been swept. The beam is controlled to brighten dots at selected points that define a character in a line of data. The beam is not brightened for at least an entire raster scan to separate one line of data from another.
Typically, a frame is divided into 40, 60 or 80 columns and 24 rows. Each column provides a character space, and each row provides a line of characters. The character space defined by a column and row count is further subdivided into a matrix of dot positions, typically 8.times.10, where each of eight horizontal dot positions in each of ten scan lines may be selectively brightened to make up a character. However, in such an arrangement, the useful dot matrix within a character space is 7.times.9, leaving a clear scan line to separate lines of characters, and a clear column at the end (or beginning) of each character space to separate characters in a line.
In a television monitor used as a data display terminal, a complete raster in a frame is divided into two fields of 262.5 scan lines per field, usually without interlacing, thus effectively providing a frame of 262 lines at the rate of 60 frames per second. For data display purposes, the output of a clock generator operating in the megahertz range is divided down to obtain a 60 Hz field (V) sync rate, and down to only about 15.75 kHz to get horizontal (H) sync rates. This chain of dividers will not only synchronize the data display with the horizontal and vertical scan of noninterlaced fields, but provide the addressing information necessary to read out into a shift register trains of binary digits, where each bit 1 will cause the beam to brighten as a line is scanned. When the entire raster of lines have been scanned, the data will have been displayed, typically in 40 columns and 24 rows of characters.
For each character space of ten raster scans per line of characters, the shift register is loaded with a new train of binary digits, which define dots to be displayed, as the previous train is shifted out into a video mixer that combines sync and blanking with the binary video into a composite signal for display. In the CRT display unit, a horizontal (H) and vertical (V) drive generator responds to the horizontal and vertical sync pulses to produce the horizontal and vertical drive signals applied to deflection coils, while the video signal from the shift register and the blanking signals are applied as the composite video to the cathode of the CRT. In that way, the beam is brightened for dots defined by 1 bits out of the shift register, and blanked for 0 bits and for line and field retrace.
To form a line of characters the clock frequency divider is used to address a random access memory (RAM) for each line of 40 characters, one character at a time in sequence. Each output character code, together with the output of a counter that counts the lines of characters, addresses a character generator implemented with a read only memory (ROM) to produce in sequence the corresponding lines of binary digits that define the characters in the row addressed. A shift register receives the binary digits in parallel, and converts them into a serial train. After the procedure has been repeated ten times for one line of 40 characters, for example, the address to the RAM is advanced to the next line of 40 characters. In that manner the output of the RAM addresses the character generator to convert the character code out of the ROM into the actual rows of dots for the characters.
The number of raster scans per field is limited to 262. For a block of 40.times.24 characters, with an 8.times.10 matrix for each character, for example, there must be 10.times.24=240 raster scans used. The rest of the time (22 raster scans.times.63.5 .mu.sec per raster scan) is not available for data display, and is instead used for field retrace.
Each dot is in actuality an ellipse with the major axis horizontal. Consequently, adjacent horizontally spaced dots tend to run together, but not fully while adjacent vertical dots do not. Space between adjacent dots in the vertical direction are more noticeable in the character than in the horizontal direction. This deficiency in both the vertical and horizontal direction provides rather low resolution of characters displayed.
A simple way to increase vertical resolution would be to use interlaced fields so that the odd field is displaced a half raster scan space, but since the data being displayed is constant until changed, the characters will appear to flicker up and down. That is quite disturbing to the viewer. It is therefore preferable to use noninterlaced fields to display data refreshed 60 times per second. The problem is to increase vertical and horizontal resolution within those constraints.